Microscopes are used in a large variety of technological applications, including in probing and testing of semiconductor microchips. With increasing metal layers and flip chip bonding, analysis of the integrated circuit (IC) can typically only be done from the backside of the chip through the silicon substrate using infrared imaging. Shrinking device geometry requires high numerical aperture (NA) lenses to resolve the transistors. In such applications, the optics plays a crucial part in device imaging, signal collection and optical probing, and in particular when the signal is optically weak.
In one existing type of optics for, e.g., probing and testing of semiconductor microchips, a SIL is placed between the object and an objective lens, with or without the use of an index matching medium between the SIL and the semiconductor microchip. The increased NA of the SIL-Objective arrangement allows higher resolution imaging, higher signal collection efficiency and smaller spot size for optical probing.
In such SIL-Objective optical arrangements, the challenges are to maintain the SIL and objective optical axis alignment when placing and focusing the SIL on the device, accommodating a small degree of tilt between device and SIL, and applying bias to eliminate the air gap at the SIL-device interface.
One existing solution in U.S. Pat. No. 7,123,035 provides the SIL attached to a bracket that is spring loaded by springs to a housing containing the objective lens. The springs extend readily inwardly from the housing and are coupled to the bracket carrying the SIL, substantially at a periphery of the SIL. Specific details as to the nature of the springs and the actual connection of the springs to the housing on the one hand, and the bracket carrying the SIL on the other hand are not provided. However, it is believed that in such a design there would be a number of practical implementation issues such as a potential vignetting effect from the springs, in particular in the biased stage upon landing of the SIL, as well as issues relating to choice of the number of springs to be used, and/or uniformity of the applied bias.
In another design described in U.S. Pat. No. 7,123,035, the SIL is fixedly attached to a SIL housing, and the objective lens is fixedly attached to an objective housing. Both the SIL housing and the objective housing are substantially cylindrical, with the SIL housing being received within the objective housing, and in a manner such that a sliding motion of the SIL housing relative to the objective housing is enabled in a biased fashion. One or more linear springs are disposed within the objective housing at its periphery, for biasing the SIL housing. Disadvantages or challenges associated with such a design include the limited, if any, ability of accommodating angular displacement between the SIL housing and the objective housing, for example as a result of landing on a surface slanted with respect to the SIL and objective housings, as well as friction between the SIL housing and the objective housing, resulting in backlash during focusing. Furthermore, since a sliding fit between the SIL housing and objective housing is required, as there is relative movement, the centering and imaging repeatability is limited by the tolerances required for the sliding fit.
A need therefore exists to provide an alternative system and method that seek to address at least one of the above-mentioned problems.